1. Technical Field
The present invention relates to a unit for monitoring the state of an assembled battery which includes a plurality of unit cells connected in series in order to output a desired voltage.
2. Background Art
For example, a hybrid vehicle includes an assembled battery which acts as a motor driving power source. Such an assembled battery obtains a high voltage by connecting, in series, a plurality of unit cells of a secondary battery (storage battery) such as, for example, a nickel-hydrogen battery, a lithium battery or the like.
For such an assembled battery, there is a need to monitor a charging state of each unit cell in order to prevent each unit cell from being over-discharged or over-charged. The monitoring of the charging state of each unit cell requires the measurement of a voltage of each unit cell and a charging/discharging current of the assembled battery. By measuring these parameters, it may be determined whether the assembled battery is in a charging condition or a discharging condition, thus detecting a charging condition of each unit cell, and further, a deterioration condition of each unit cell may be determined by calculating a resistance value of each unit cell.
Typically, a control system for an assembled battery includes a plurality of high voltage side controllers connected respectively to charging/discharging circuits of the assembled battery and a low voltage side controller electrically isolated from the plurality of high voltage side controllers. Each of the plurality of high voltage side controllers is provided to correspond to each unit cell and measures a voltage of each unit cell. The measured voltage of each unit cell is output to the low voltage side controller via a non-contact connector such as a photo coupler or the like. The low voltage side controller measures a charging/discharging current of each of the assembled battery by means of a non-contact type current sensor. The measured charging/discharging current of each of the assembled battery, along with the voltage of each unit cell input from each of the plurality of high voltage side controllers, is output, either as it is or after being subjected to a process, to a high level computer equipped within the vehicle.
In this manner, when the high voltage side controllers measure voltages of unit cells and the low voltage side controller measures the charging/discharging current of the assembled battery, there is a need to synchronize both measurement timings. One reason for this may include a frequent alternation between vehicle acceleration and deceleration in the hybrid vehicle.
Such an alternation between vehicle acceleration and deceleration causes an alternation between powering conditions and regenerative conditions of a motor generator and hence an alternation between the charging condition and discharging condition of the assembled battery. Accordingly, if the measurement timing of voltage of the unit cells is not in synchronization with the measurement timing of charging/discharging current of the assembled battery, a voltage of the unit cell measured during discharging of the assembled battery may be actually mistaken as a voltage during charging from a result of measurement of the charging/discharging current of the assembled battery, which may result in misrecognition of charging conditions of the batteries. In addition, such timing asynchronization may result in incorrect determination of resistance of the unit cell from the voltage of the unit cell and the charging/discharging current of the assembled battery.
So, if a measurement voltage of each unit cell is individually input from each of the high voltage side controllers to the low voltage side controller, the above-mentioned synchronization can be obtained by the low voltage side controller measuring the charging/discharging current of the assembled battery in synchronization with an input timing of the measurement voltage of the unit cell (for example see JP-A-2009-50085).
The above-mentioned synchronization method is based upon the premise that the low voltage side controller can determine from which of the high voltage side controllers the measurement voltage of the unit cell is input. For this reason, the low voltage side controller is required to output a voltage measurement command to each of the high voltage side controllers individually and determine from which of the high voltage side controllers the measurement voltage of the unit cell in response to this voltage measurement command is input.
Accordingly, if the low voltage side controller outputs a common voltage measurement command to all of the high voltage side controllers and, in response to this command, each of the high voltage side controllers outputs a measurement voltage of a corresponding unit cell to the low voltage side controller, it becomes difficult to synchronize the measurement timing of the charging/discharging current of the assembled battery with the measurement timing of the voltage of each unit cell. This is because the low voltage side controller can not determine from which of the high voltage side controllers the measurement voltage of the unit cell is input to the low voltage side controller, based on the fact that the voltage measurement command is common to all of the high voltage side controllers.